Technical Field
The technical field relates generally to motor vehicle compressed air systems and more particularly to control over an internal combustion engine driven compressor in a compressed air system.
Description of the Technical Field
In order to meet demands for improved motor vehicle fuel economy engineers have taken steps to reduce energy consumption attributable to parasitic motor vehicle systems such as power steering systems, air conditioning systems and pneumatic systems. Pneumatic systems have long been used with motor vehicle air brake systems and recent developments have expanded the scope of their application on vehicles. A pneumatic system architecture utilizes a piston in cylinder type compressor pump which is mechanically engaged to the thermal/internal combustion engine. The compressor pump feeds a compressed air storage system (represented here as a simple tank) through a check valve. The compressor draws air through an intake valve which may be closed to allow compression of air during a piston compression stroke and opened to allow air to be drawn into the compressor during a piston down stroke.
Air pressure in the storage system is kept within minimum and maximum bounds by discontinuing delivery of air to the storage system when pressure in the storage system reaches a maximum allowed level and resuming air delivery when pressure falls to a minimum allowed value or is below the minimum allowed value, as may occur on engine start where the vehicle has sat for some time. In simple systems, when fully pressurized, the compressor pump may continue to operate and compressed air discharged to the environment. In such an arrangement the compressor pump absorbs up to two or more horsepower from the engine.
In order to avoid the waste of driving a compressor pump when pressure in the storage system is at its maximum allowed value, some systems provide for the compressor to be mechanically disengaged or unloaded. Mechanical disengagement can involve use of a clutch and can impose a penalty in terms of added weight and relative complexity compared with systems for unloading the compressor. Unloading a compressor interrupts the compression cycle. To do this a compressor unloader holds the compressor's intake valve open when compressed air storage facilities are fully pressurized. The air compressor piston continues to cycle, but simply draws and pushes air in and out through the intake valve. Little or no compression of air occurs and the load imposed by the air compressor on the engine is reduced to relatively minor frictional losses.
The compressor unloader itself is a piston which is displaced to hold open the compressor intake valve. Pneumatic compressor unloaders utilize compressed air from system storage the flow of which is triggered when air pressure in storage reaches its maximum allowed value. When air pressure in the storage system falls to or is below the minimum allowed value the compressor unloader is discharged and the air compressor intake valve operates to allow the air compressor to compress air.